High voltage transformer



Dec.'28, 1943. A. MEYERHANS ETAL .meer

HIGH VOLTAGE TRANSFORMER Filed May 15, 1940 AM/A750 PAPER Patented Dec.28, 1943 UNlTED Sl TES idii'llfgltl'l' OFFICE HIGH VOLTAGE TRANSFORMER.application May 13, 1940, Serial No. 334,914 ln Germany May 15, 1939(Cl. 17E-362) l0 Claims.

The construction of oil-immersed power transformers for high voltages,with rigid insulation of high value insulating material between the highand low Voltage winding in the form of a cylinder against which theactive parts abut with practically no intermediate spaces, isaccompanied by special dilculties as regards the insulation. It is notonly a question of preventing flashov'ers between parts having differentpotentials, but also the electric eld which forms at the points wherethe insulating cylinder passes between windings and/or magnetic coreparts at diiferent potentials must be influenced in such a manner thatthe field strength decreases uniformly in the direction of thetransformer casing or yoke and that a maximum allowable longitudinalvoltage gradient in the remaining cil layers between the rigidinsulating parts is not exceeded. For convenience of description, theterm active parts will be employed in the following specification andthe claims to identify the electrically conductive and the magneticelements (windings and core) of the transformer between which, as iswell known, substantial differences' in potential are established. y

In addition to overcoming these difficulties it is also the object ofthe invention to reduce the overall dimensions of the transformer and toachieve a saving in space, weight and production costs, when comparedwith the designs hitherto used where the insulation consists of layerscil and walls of rigid insulating material.

The invention relates more particularly to the insulation cf highVoltage transformers of the type shown in the patents of AugustMeyerhans l Nos. 2.279.239 April 7, i942, and 2,288,201 June 30, 1942.

The invention thus concerns a transformer for high voltages with rigidinsulation of a high value insulating material between the high and lowvoltage winding in the form of a cylinder against which the active partsabut withpractically no intermediate spaces, whereby the aforementioneddifliculties are overcome and the desired is achieved. According to theinvention the insulatN ing cylinder is made to extend beyond the endsurfaces of the'active parts of the'transfoi vr in an axial direction inorder to prevent dashovers between parts having different potentials,and furthermore electrodes are provided on these end surfaces on bothsides of the insulating cylinder which at least partlyoverlap thecylindrical surfaces of the active parts facing the insulatingcylinder'and the end surfaces of these parts, in-

sulation inserts of rigid insulating material being also provided whichfill the spaces formed between the insulating cylinder and theelectrodes and also overlap the active parts, so that the distributionof the electric field forming at the points where the insulatingcylinder emerges from the active parts is such that in the gaps betweenthe insulation inserts and the insulating cylinder a maximum allowablelongitudinal voltage gradient is not exceeded.

Several constructional examples cf the invention are illustrateddiagrammatically in longitudinal section in Figs. 1-3 of theaccompanying drawing. Fig. 1 snows the insulation between parts of theiron core, and between the core parts and the transformer casing, whilstFigs. 2 and 3 show the insulation between the high and low voltagewinding and between the latter and the yoke of the iron core.

In all the figures, l indicates the insulating cylinder made of highquality paper layers disposed between the high voltage winding 2 whichsurrounds the central core part il and the concentric low voltagewinding 3, the active parts (coils, iron) abutting against the cylinderpractically without any clearance. The insulating cylinder l extendsbeyond the end surfaces of the windings of the transformer in an axialdirection towards the yoke 4, 4a and the transformer casing, whichlatter is not shown in the drawing. Electrodes l, t are arranged on theend surfaces of the outer Aactive parts of the transformer on both sidesof the insulating cylinder l, these electrodes overlapping at least partof the cylindrical surfaces of the active parts facing the insulatingcylinder, insulation inserts l, 3 of rigid stratified insulatingmaterial (paper) being provided to nl] the free spaces between theinsulating cylinder l and the electrodes 5, B and thus also overlappingthe active parts. The axially elongated insulating cylinder i and theinsulation inserts l, simplify the building of the transformer and alsothe insertion and removal of the coils, but necessitate gaps betweenthese'parts which are filled with oil. The electrodes 5, Si musttherefore be given such a shape that the distribution of the electricfield which forms at the points where the insulating cylinder emergesfrom active parts is such that in the gaps between the insulationinserts and the cylinder a maximum allow able longitudinal voltagegradient -is not exceeded. By this means the voltage'drop in the gapboth in the longitudinal and transverse direction is reduced to a valuewhich lies below the critical limit for flash-overs in oil.

In the constructional examples shown in Figs. 1 and 2 the electrodes 5,6 are drawn into the active parts of the transformer and arranged toform in close contact with the insulating cylinder l screens for the endsurfaces of the active parts. The electrodes 5, 6 have a conical part,as shown in the drawing, the distance of which from the point of contactwith the insulating cylinder increases only very gradually, andconnected therewith is a iiat part which overlaps the end surfaces ofthe active parts of the transformer.

Fig. l illustrates the application of electrodes il, 6 for controllingthe field and the voltage gradient at the ends of the insulatingcylinder i and also of insulation inserts l, 8 for insulatn ing parts ofthe iron core, namely the yoke ii, from each other and from thetransformer casing which is not shown in the drawing. The high voltagewinding 2 is surrounded by the insulating cylinder l. The yoke comprisesa number ci separate parts, namely the high voltage part resting on thecore 4' and the part fla which forms the yoke plate and has a lowvoltage or earth potential. Between the yoke and the low voltage winding3 there is a metallic spacing ring 9 which is provided with a coveringof insulation. rIhe yoke parts a are stepped towards their end surfacesand together with the electrodes 5, G have such a form that thelongitudinal gradient in the gap between the insulating cylinder i andthe insulation inserts l, ii does not exceed a definite maximum value sothat creeping discharges and oil hash-overs can no longer occur. Theinsulation inserts between the cylinder and the electrodes are in theform of angle rings one limb of which abuts against the insulatingcylinder and is tapered so as to penetrate as far as possible betweenthe stepped yoke parts or the electrodes and the insulating cylinder,whilst the other limb of each ring covers the yoke parts and is sodimensioned that the field forming at the yoke parts causes theinsulation. to be uniformly relieved, The angle rings are preferablymade of stratied insulating material (paper), the direction of thelayers being arranged at least approximately perpendicular to thedirection oi the electric eld.

Fig. 2 illustrates the application of the electrodes 5, for controllingthe i'ield and the voltage gradient and also the use of insulationinserts l, ii for insulating the high and low voltage windings 2 and 3respectively from each other and from the yoke li. In this case the lowvoltage winding 3 is surrounded by the insulating cylinder l. The endsurfaces of the windings are stepped to conform with the shape of theelectrodes. The construction of the insulating cylinder, electrodes andinsulation inserts is identical with that shown in Fig. 1 and theireiect is also the same. The electrodes 5, 6 are preferably pro-videdwith a rounded edge orbead it at the ends which overlap the windings 2,3 in order to prevent corona dischages. In order to supplement theinsulation an optional nurnber of insulating rings ii of high qualityinsulating material can be provided which are mutually supported byspacers l2, these latter also supporting the rings Il against theinsulation inserts 1, S and the yoke li respectively. Similar insulatingelements I3 can also be provided between the windings 2, 3 and theelectrodes l, 3. The high voltage Winding 2 and the low voltage winding3 are provided with axial longitudinal cooling ducts.

Instead of employing special electrodes ashown in the constructionalexamples of Figs. and 2 it is of course possible to construct th endsurfaces of the active transformer parte s that they act as electrodesfor reducing electric eld which forms at the points wher the insulatingcylinder emerges from the active parts. It is also possible to use aselectrodes the protective rings which are often provided on the activeparts, these rings being either of metal or insulating material with ametallized surface. If such electrodes are in conjunction with theinsulation inserts according to the constructional examples illustratedin Figs. l and 2, it is preferable to insert additional insulationconsisting of layers of insulating material between the electrodes andthe angular insulation inserts` Fig. 3 shows a constructional examplewhere this last mentioned arrangement is employed. The insulatingcylinder l is again located be tween the high voltage winding 2 and thelow voltage winding 3. The electrodes are formed by protective rings ofinsulating material with metallized surfaces Hl, l5 arranged on the endsurfaces of the windings and by the windings 2, 3 which are steppedtowards the end surfaces. Additional insulation elernents iii, il aredisposed between the electrodes Hl, l and the insulation inserts l, 3.The insulation inserts l, il in this case do not penetrate to the pointwhere the stepping of the windings 2, 3 commences, but the additionalinsulation elements i6, l'l extend in an axial direction into thewindings and are stepped in such a manner that the ends of these steppedlayers can be bent around these coil surfaces which are opposite to t1 eend surface of the winding. Care must, however,v be taken that thelongitudinal cooling ducts for the windings are kept free. Theadditional insulation elements i5, il lying between the electrodes lil,i5 and the insulation inserts 1, il can overlap the electrodes radiallyor their edges can be bent over the electrodes,

as shown in Fig. 3.

The effect of the arrangement shown in Fig. 3 is the same as thatdescribed in connection with the constructions illustrated in Figs. l 2.The object of bending the stepped layers ci the additional insulationelements i5, il around the coils of the windings is to prevent theoccurrence of partial discharges from the coils. In order to increasethe leakage path between the sides of the insulating cylinder l and theinsulation inserts l, 8 respectively, he surface area can be increasedby undulating, folding or roughening. It is also possible to taper theends of the insulating cylinder i which extend .beyond the windings 2, 3along the length which projects into the additional insulation elementsThe various arrangements of the insulation and the diiferent forms ofelectrodes illustrated in Figs. 1 3 can be interchanged, or combined asdesired, in order to insulate the active parts (windings, iron) of atransformer from cach other or from other parts. For instance the highvoltage winding or the iron part can be insulated as shown in Fig. 2 andthe low voltage winding as shown in Fig. 3, or vice versa.

Having new particularly described and ascertained the nature of our saidinvention and in what manner the same is to be performed, We declarethat what we claim is:

1. In a transformer for high voltages comprising active parts includingconcentric high and low tension windings and core members, an insulatingcylinder disposed between and substantially abutting said windings, saidinsulating cylinder extending axially beyond the end surfaces of thewindings, annular metallic electrode members disposed externally andinternally of said cylinder adjacent the projecting end thereof, saidelectrode members each having a. skirt portion extending along thesurface of said cylinder between the same and the adjacent portions ofthe active parts and having a flared portion extending from said skirtportion away from the surface of said cylinder in a curved path andoverlapping the end surfaces of said portions of the active parts,annular insulating inserts fitting externally and internally theprojecting end of said cylinder and each having a tapered dependingskirt portion complemental to the shape of the respective electrodemembers and filling the space between the same and the respectivesurfaces of said insulating cylinder, said insulating inserts eachhaving a flange portion substantially coextensive with the flaredportion of said electro-des and also overlapping the end surfaces ofsaid portions of the active parts of the transformer.

2. A transformer as claimed in claim 1, characterised by the featurethat the electrodes extend at least partly into the active parts of thetransformer and are constructed so as to lie in close contact with theinsulating cylinder and to form screens for the end surfaces of theactive parts.

3. A transformer as claimed in claim l, characterised by the useelectrodes which control the field and the voltage gradient at the endsof the insulating cylinder and also insulation inserts in the spacebetween the insulating cylinder and the electrodes for insulating thehigh voltage winding from the low voltage winding and these latter fromthe yoke.

4. A transformer as claimed in claim 1, characterised by the featurethat the insulation inserts,l in the intermediate space between theinsulating cylinder and the electrodes are in the form of an angle ringof which the limb abutting against the insulating cylinder is tapered.

5. A transformer as claimed in claim 1, characterised by the featurethat the insulation inserts in the intermediate space between theinsulating cylinder and the electrodes are each in the form of an anglering made of stratified insulating material, the direction of theinsulating layers being at least approximately perpendicular to thedirection of the electric field.

6. A transformer as claimed in claim 1, characterised by the featurethat the electrodes are in the form of protective rings made of metal ormetallized insulating material.

7. A transformer as claimed in claim 1, characterised by the featurethat an additional insulation of stratified insulating material isdisposed between the electrodes and the angle-shaped insulation inserts.

8. A transformer as claimed in Claim 1, characterised by the featurethat an additional insulation of stratified insulating material isdisposed between the electrodes and the insulation inserts and that thelayers of the additional insulation are stepped down in the axialdirection of the active parts, the ends of the layers being bent roundthe coil surfaces opposite to the end surfaces.

9. A transformer as claimed in claim 1, characterised by the featurethat the surface paths facing each other on the insulation inserts andthe insulating cylinder are enlarged for the leakage currents.

l0. A transformer as claimed in claim l, characterised by the featurethat the end surfaces of the active parts of the transformer immediatelyadjacent the electrodes are stepped in a radial direction to correspondwith the shape of the electrodes.

AUGUST MEYERHANS.

RUDOLF SCHRLI.

